ensure headers in playlist are sized according to the font

Summary:
The header size eed to take into account the height of the font such
that it does not overflow other items.

This also fixes the album cover not looking centered with respect to
the header background.

Test Plan: The layout is now handling nicely different font sizes.

Reviewers: astippich, januz, ognarb

Differential Revision: https://phabricator.kde.org/D15322